516 GENITAL ORGANS OF THE MARE 



Structure. — The intrinsic muscular coat consists of longitudinal and circular 

 layers of unstrii^cd fibers. Between these is a laj'cr of areolar tissue which contains 

 a rich venous plexus. The mucous membrane is thrown into longitudinal folds 

 when the canal is closed; it is highly elastic, and is covered with stratified epithe- 

 lium. 



The urethral muscle (M. compressor urethrae) embraces the urethra and is 

 continued Ijackward for some distance under the constrictor vestibuli. It is 

 covered by a fibro-elastic membrane. 



THE MAMMARY GLANDS 



The mammary glands (Glandulse lactiferae) are modified cutaneous glands 

 which are so closely associated functionally with the genital organs as to be con- 

 sidered accessory to them. 



In the mare they are two in number, and are placed on either side of the median 

 plane in the inguinal region. Each gland has the form of a short cone, much com- 

 pressed transversely, and having a flat inner surface. It consists of the glandular 

 mass or body of the gland (Corpus mammae) and the teat or nipple (Papilla mam- 

 mae). The base is related to the abdominal wall, to which it is attached by areolar 

 tissue, which contains a venous plexus, the superficial inguinal lymph glands, 

 and a variable amount of fat. The apex is constituted by the teat, which is also 

 flattened transversely and varies in length from one to two inches (ca. 2.5 to 5 cm.). 

 Between the bases of the teats is the intermammar}'' groove. On the apex of each 

 teat two or three small orifices are placed close together; these are the openings 

 of the lactiferous ducts. 



Structure. — The skin over the glands is thin, pigmented, chiefly hairless, and 

 supplied with numerous large sebaceous and sweat glands. Under this are two 

 layers of fascia except on the teats. The superficial fascia presents no special 

 features. The deep fascia consists of two laminae detached from the abdominal 

 tunic which descend on either side of the median plane, forming a septum between 

 the two glands, and constituting their Hgamentum suspensorium. 



These laminae are separated by a layer of ai-eolar tissue, so that it is possible to remove one 

 gland if diseased by careful dissection between the layers of the septum. 



The gland substance or parenchyma is pinkish gray in color, and of firmer 

 consistence than the fat which is found around and witliin the gland. It is inclosed 

 by a fibro-elastic capsule which sends inward numerous trabeculae; these form the 

 interstitial tissue, and divide the gland into lobes and lobules. In the latter are 

 the secretory tubules and alveoli, which unite to form the larger ducts. Each lobe 

 has a duct, which opens at the base of the teat into a s])ace called the lactiferous 

 sinus (Sinus lactiferus) ; this is lined with mucous mendjrane, and from it tv,o 

 (or three) lactiferous ducts (Ductus lactiferi) pass through the extremity of the 

 nipple. These ducts are lined with a non-glandular mucous membrane, which is 

 covered with stratified squamous epithelium. They ai"e surroimded by unstriped 

 muscular tissue, the bulk of the fibers being arranged in a circular manner to form 

 a sphincter. 



'Ili(> size and form of llic maiiuiiiiry jrlands arc .sul)j('ct to imicli \'ariation. In the young; 

 subject, l)eforc prcfinancy, llicy arc small and contain little gland tissue. During the latter 

 part of gestation, and cspcciallj^ during lactation, they increase greiltly in size, and the gland 

 tissue is liiglily developed. After lactation the secretory structun-s uii(l(>rgo marked involution, 

 and the gland is much reduced in size. The relative amounts of gland substance and interstitial 

 tissue vary greatly; in some cases a gland of considerable size contains little secretory tissue and 

 is consequently functionally deficient. 



Vessels and Nerves. — The arteries are derived from the external pudic 

 artery, which enters the gland at the posterior part of its base. The veins form a 



